res_pjsip_diversion: resolve race condition between Diversion header processing and...

res_pjsip_diversion: resolve race condition between Diversion header processing and redirect

Based on the firing order of the PJSIP call-backs on a redirect, it was possible for
the Diversion header to not be included in the outgoing 181 response to the UAC and
the INVITE to the UAS.

This change moves the Diversion header processing to an earlier PJSIP callback while also
preventing the corresponding update that can cause a duplicate 181 response when processing
the header at that time.

Resolves: #1349